Cedar-apple rust

Natural History
Mature cedar-apple rust galls on a redcedar tree
Photo credit: Department of Plant Pathology, University of Missouri-Columbia

Cedar-apple rust is caused by a fungus (Gymnosporangium juniperi-virginianae) but unlike many diseases, this fungus requires two different trees (hosts) to complete its life cycle - a redcedar tree and an apple or hawthorn tree. The pathogen produces tumor-like growths (galls) on branches of redcedar trees but causes little damage to these trees. The disease also causes damage to apple or hawthorn leaves and fruit. Cedar-apple rust is not uncommon in Florida but is not considered much of a threat. The disease causes great concern in states where apple orchards are big cash crops.

The life cycle starts in late summer when spores are produced on the leaves of hawthorn or apple trees. The spores are carried by wind to redcedar trees. The spores eventually develop into galls. The galls produce bright orange, jelly-like horns that create new spores. These new spores are carried by wind to apple and hawthorn trees where the cycle starts again.

Note: The common names used for both the disease and the trees can make this subject a little confusing. In spite of its name, cedar-apple rust doesn't actually infect the true cedars, which have needles like pines and are in the genus Cedrus. What is called a "redcedar" in the eastern U.S. is actually a type of juniper and has branches covered in tiny green scales. Cedar-apple rust only infects these redcedars and other related species in the genus Juniperus. It would be more accurate to change the name of the disease to "juniper-apple rust," but that isn't likely to happen anytime soon.


 

Identifying Characteristics

Identifying the disease: Yellow spots that later turn brown are found on apple and hawthorn leaves. These spots cause the leaves to curl up and under. On redcedar trees, look for reddish-brown, round galls and, in the spring, the orange, jelly-like horns that develop from the galls.
Susceptible trees: The primary hosts of cedar-apple rust are trees in the rose family, including apple (Malus spp.), hawthorn (Crataegus spp.), and pear (Malus spp.). The alternate hosts are eastern redcedars (Juniperus virginiana) and southern redcedars (Juniperus silicicola). The disease is most commonly found in areas where apples or hawthorn trees are close to redcedar trees.

 

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